Process for pyrolyzing coal using a recycled hydrogen donor

ABSTRACT

A process for pyrolyzing coal using a recycled hydrogen donor includes introducing a coal feed to a pyrolysis zone and heating the coal feed to a temperature of about 300° C. in the absence of hydrogen. A hydrogen donor solvent is introduced to the pyrolysis zone after the coal feed is heated to about 300° C., and the temperature of the coal feed and the hydrogen donor solvent is increased to about 475° C., while increasing a pressure in the pyrolysis zone to at or above a vapor pressure of the hydrogen donor solvent. At least an aromatic hydrocarbon rich fraction is separated from the coal tar stream and hydrogenated. The hydrogenated aromatic hydrocarbon rich fraction is recycled to the pyrolysis zone as the hydrogen donor solvent.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 61/905,931 filed on Nov. 19, 2013, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Many different types of chemicals are produced from the processing of petroleum. However, petroleum is becoming more expensive because of increased demand in recent decades.

Therefore, attempts have been made to provide alternative sources for the starting materials for manufacturing chemicals. Attention is now being focused on producing liquid hydrocarbons from solid carbonaceous materials, such as coal, which is available in large quantities in countries such as the United States and China.

Pyrolysis of coal produces coke and coal tar. The coke-making or “coking” process consists of heating the material in closed vessels in the absence of oxygen to very high temperatures. Coke is a porous but hard residue that is mostly carbon and inorganic ash, which can be used in making steel.

Coal tar is the volatile material that is driven off during heating, and it comprises a mixture of a number of hydrocarbon compounds. It can be separated to yield a variety of organic compounds, such as benzene, toluene, xylene, naphthalene, anthracene, and phenanthrene. These organic compounds can be used to make numerous products, for example, dyes, drugs, explosives, flavorings, perfumes, preservatives, synthetic resins, and paints and stains. The residual pitch left from the separation is used for paving, roofing, waterproofing, and insulation.

Pyrolyzing a coal feed produces both coke and coal tar. It is desirable to be able to control the ratio of these pyrolysis products. Additionally, it is desirable to reuse hydrogenated compounds to increase the amount of hydrogen present in the coal tar stream.

Thus, there is a need for a process for pyrolyzing coal using a recycled hydrogen donor molecule. There is also a need for a process for controlling a volume of a coke output during coal pyrolysis.

SUMMARY OF THE INVENTION

In a first aspect, a process for pyrolyzing coal using a recycled hydrogen donor includes introducing a coal feed to a pyrolysis zone and heating the coal feed to a temperature of about 300° C. in the absence of hydrogen. A hydrogen donor solvent is introduced to the pyrolysis zone after the coal feed is heated to about 300° C., and the temperature of the coal feed and the hydrogen donor solvent is increased to about 475° C., while increasing a pressure in the pyrolysis zone to at or above a vapor pressure of the hydrogen donor solvent. The process further includes increasing the temperature of the coal feed and the hydrogen donor solvent to about 600° C. while maintaining the pressure in the pyrolysis zone at or above the vapor pressure of the hydrogen donor solvent to produce a coke stream and a coal tar stream, and reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1000° C. to about 2,000° C. At least an aromatic hydrocarbon rich fraction is separated from the coal tar stream and hydrogenated. The hydrogenated aromatic hydrocarbon rich fraction is recycled to the pyrolysis zone as the hydrogen donor solvent.

In another aspect, a process for controlling a volume of a coke output during coal pyrolysis includes introducing coal to a pyrolysis zone and heating the coal to a temperature of about 300° C. in the absence of hydrogen. A hydrogen donor solvent is then introduced to the pyrolysis zone. The temperature of the coal and the hydrogen donor solvent is increased to a temperature of about 475° C., while increasing a pressure in the pyrolysis zone to at or above a vapor pressure of the hydrogen donor solvent. The process further includes increasing the temperature of the coal and the hydrogen donor solvent to about 600° C. while maintaining the pressure in the pyrolysis zone to produce a coke stream and a coal tar stream, and reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1,000° C. to about 2,000° C. The amount of hydrogen donor solvent introduced into the pyrolysis zone relative to an amount of coal introduced to the pyrolysis zone is varied to control an amount of coke produced in the coke stream.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates one embodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE shows one embodiment of a coal pyrolysis process 5. A coal feed 10 can be sent to a pyrolysis zone 15, such as a coking oven. Alternatively, the coal feed 10 can be sent to a gasification zone 20, or be split into two parts and sent to both the pyrolysis zone 15 and the gasification zone 20.

In the pyrolysis zone 15, the coal is heated to high temperature in the absence of oxygen to produce a coke stream 25 and a coal tar stream 30. In particular, the coal feed 10 is first heated to a temperature of about 300° C. in the absence of hydrogen. This initial heating begins to drive off volatile compounds.

Once the coal has been heated to about 300° C., a hydrogen donor solvent 75 is introduced into the pyrolysis zone 15. The temperature in the pyrolysis zone 15 is then increased to bring the coal feed 10 and the hydrogen donor solvent 75 to a temperature of about 475° C. The pressure in the pyrolysis zone 15 is also increased so that the pressure in the pyrolysis zone is at or above the vapor pressure of the hydrogen donor solvent 75. This increase in temperature and pressure results in coal tar evolving in the pyrolysis zone.

The temperature in the pyrolysis zone 15 is further increased to bring the coal feed 10 and hydrogen donor solvent 75 to a temperature of about 600° C. while maintaining the pressure in the pyrolysis zone. During this heating process, the coke stream 25 and the coal tar stream 30 are formed. Once no further coal tar is evolved from the pyrolysis zone 15, the pressure in the pyrolysis zone 15 is reduced to approximately atmospheric pressure, and the temperature further increases. The temperature in the pyrolysis zone 15 is increased to heat the coal to a temperature in the range of about 1,000° C. to about 2,000° C., and preferably to a temperature of about 1,100° C. The coal is maintained at this temperature until it has been fully converted.

The coke in the coke stream 25 produced in the pyrolysis zone 15 can be used in other processes, such as the manufacture of steel.

The coal tar stream 30, which comprises the volatile components from the coking process, can be sent to a contamination removal zone 35, if desired. Relative volumes of the coke stream 25 and the coal tar stream 30 can be controlled by adjusting the amount of hydrogen donor solvent 75 introduced into the pyrolysis zone 15 to vary a molar ratio of hydrogen to carbon in the pyrolysis zone 15. In particular, it has been found that a molar ratio of hydrogen to carbon in the range of 0.5 to 0.7 produces a particularly desirable output, increasing the amount of coal tar produced relative to the amount of coke produced. However, other ratios are contemplated as being within the scope of the invention.

The optional contaminant removal zone 35 removes one or more contaminants from the coal tar stream 30 or another process stream, and may be located at various positions along the process depending on the impact of the particular contaminant on the product or process and the reason for the contaminant's removal, as described further below. For example, the contaminant removal zone 35 can be positioned upstream of the separation zone 45. Some contaminants have been identified to interfere with a downstream processing step or hydrocarbon conversion process, in which case the contaminant removal zone 35 may be positioned upstream of the separation zone 45 or between the separation zone 45 and the particular downstream processing step at issue. Still other contaminants have been identified that should be removed to meet particular product specifications. Where it is desired to remove multiple contaminants from the hydrocarbon or process stream, various contaminant removal zones may be positioned at different locations along the process. In still other approaches, a contaminant removal zone 35 may overlap or be integrated with another process within the system, in which case the contaminant may be removed during another portion of the process, including, but not limited to the separation zone 45 or the downstream hydrocarbon conversion zone. This may be accomplished with or without modification to these particular zones, reactors or processes. While the contaminant removal zone 35 is often positioned downstream of the hydrocarbon conversion reactor, it should be understood that the contaminant removal zone 35 in accordance herewith may be positioned upstream of the separation zone 45, between the separation zone 45 and the hydrocarbon conversion zone, or downstream of the hydrocarbon conversion zone or along other streams within the process stream, such as, for example, a carrier fluid stream, a fuel stream, an oxygen source stream, or any streams used in the systems and the processes described herein. The contaminant concentration is controlled by removing at least a portion of the contaminant from the coal tar stream 30. As used herein, the term removing may refer to actual removal, for example by adsorption, absorption, or membrane separation, or it may refer to conversion of the contaminant to a more tolerable compound, or both.

The decontaminated coal tar feed 40 is sent to a separation zone 45 where it is separated into two or more fractions. Coal tar comprises a complex mixture of heterocyclic aromatic compounds and their derivatives with a wide range of boiling points. The number of fractions and the components in the various fractions can be varied as is well known in the art. A typical separation process involves separating the coal tar into four to six streams. For example, there can be a fraction comprising NH₃, CO, and light hydrocarbons, a light oil fraction with boiling points between 0° C. and 180° C., a middle oil fraction with boiling points between 180° C. to 230° C., a heavy oil fraction with boiling points between 230 to 270° C., an anthracene oil fraction with boiling points between 270° C. to 350° C., and pitch.

The light oil fraction contains compounds such as benzenes, toluenes, xylenes, naphtha, coumarone-indene, dicyclopentadiene, pyridine, and picolines. The middle oil fraction contains compounds such as phenols, cresols and cresylic acids, xylenols, naphthalene, high boiling tar acids, and high boiling tar bases. The heavy oil fraction contains benzene absorbing oil and creosotes. The anthracene oil fraction contains anthracene. Pitch is the residue of the coal tar distillation containing primarily aromatic hydrocarbons and heterocyclic compounds.

As illustrated in the figure, the coal tar feed 40 is separated into gas fraction 50 containing gases such as NH3 and CO as well as light hydrocarbons such as ethane, hydrocarbon fractions 55, 60, and 65 having different boiling point ranges, and pitch fraction 70.

Suitable separation processes include, but are not limited to fractionation, solvent extraction, distillation, and aromatic extraction.

One or more of the fractions 50, 55, 60, 65, 70 can be further processed, as desired. As illustrated, a fraction 65 can be sent to a hydrogenation zone 80. The fraction 65 is an aromatic hydrocarbon-rich fraction including hydrocarbons that have an initial boiling point in the range of about 180° C. to about 270° C. The aromatic hydrocarbon-rich fraction 65 preferably includes polycyclic aromatic hydrocarbon compounds, such as naphthalene and alkylnaphthalenes. Hydrogenation involves the addition of hydrogen to hydrogenatable hydrocarbon compounds. The fraction 65 is introduced into the hydrogenation zone 80 and contacted with a hydrogen-rich gaseous phase and a hydrogenation catalyst in order to hydrogenate at least a portion of the hydrogenatable hydrocarbon compounds. For example, naphthalene can be hydrogenated to form tetralin and decalin. The catalytic hydrogenation zone may contain a fixed, ebulated or fluidized catalyst bed. The hydrogenation zone 80 is typically at a pressure from about 689 kPag (100 psig) to about 13,790 kPag (2,000 psig) with a maximum catalyst bed temperature in the range of about 177° C. (350° F.) to about 454° C. (850° F.). The liquid hourly space velocity is typically in the range from about 0.2 hr⁻¹ to about 10 hr⁻¹ and hydrogen circulation rates from about 200 standard cubic feet per barrel (SCFB) (35.6 m³/m³) to about 10,000 SCFB (1,778 m³/m³).

The hydrogenated aromatic hydrocarbon-rich fraction is then recycled to the pyrolysis zone 15 as the hydrogen donor solvent 75. The recycling can be continuous as shown in the FIGURE, or the hydrogenated aromatic hydrocarbon-rich fraction can be stored for later use as a hydrogen donor solvent, depending on the needs of the process.

One or more hydrocarbons of the fractions 50, 55, 60, 70 can be subject to further downstream processing at one or more hydrocarbon conversion zones (not shown). Additionally, the aromatic hydrocarbon rich fraction 65 can be split prior to the hydrogenation zone 80, such that a portion of the fraction 65 is subject to downstream processing. Suitable hydrocarbon conversion zones include, but are not limited to, hydrotreating zones, hydrocracking zones, fluid catalytic cracking zones, alkylation zones, transalkylation zones, and oxidation zones.

Hydrotreating is a process in which hydrogen gas is contacted with a hydrocarbon stream in the presence of suitable catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen, oxygen, and metals from the hydrocarbon feedstock. In hydrotreating, hydrocarbons with double and triple bonds may be saturated. Aromatics may also be saturated. Typical hydrotreating reaction conditions include a temperature of about 290° C. (550° F.) to about 455° C. (850° F.), a pressure of about 3.4 MPa (500 psig) to about 6.2 MPa (900 psig), a liquid hourly space velocity of about 0.5 hr⁻¹ to about 4 hr⁻¹, and a hydrogen rate of about 168 to about 1,011 Nm³/m³ oil (1,000 to 6,000 scf/bbl). Typical hydrotreating catalysts include at least one Group VIII metal, preferably iron, cobalt and nickel, and at least one Group VI metal, preferably molybdenum and tungsten, on a high surface area support material, preferably alumina. Other typical hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum.

Hydrocracking is a process in which hydrocarbons crack in the presence of hydrogen to lower molecular weight hydrocarbons. Typical hydrocracking conditions may include a temperature of about 290° C. (550° F.) to about 468° C. (875° F.), a pressure of about 3.5 MPa (500 psig) to about 20.7 MPa (3,000 psig), a liquid hourly space velocity (LHSV) of about 1.0 to less than about 2.5 hr⁻¹, and a hydrogen rate of about 421 to about 2,527 Nm³/m³ oil (2,500 to 15,000 scf/bbl). Typical hydrocracking catalysts include amorphous silica-alumina bases or low-level zeolite bases combined with one or more Group VIII or Group VIB metal hydrogenating components, or a crystalline zeolite cracking base upon which is deposited a Group VIII metal hydrogenating component. Additional hydrogenating components may be selected from Group VIB for incorporation with the zeolite base.

Fluid catalytic cracking (FCC) is a catalytic hydrocarbon conversion process accomplished by contacting heavier hydrocarbons in a fluidized reaction zone with a catalytic particulate material. The reaction in catalytic cracking is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. The process typically employs a powdered catalyst having the particles suspended in a rising flow of feed hydrocarbons to form a fluidized bed. In representative processes, cracking takes place in a riser, which is a vertical or upward sloped pipe. Typically, a pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts hot fluidized catalyst and is vaporized on contact with the catalyst, and the cracking occurs converting the high molecular weight oil into lighter components including liquefied petroleum gas (LPG), gasoline, and a distillate. The catalyst-feed mixture flows upward through the riser for a short period (a few seconds), and then the mixture is separated in cyclones. The hydrocarbons are directed to a fractionator for separation into LPG, gasoline, diesel, kerosene, jet fuel, and other possible fractions. While going through the riser, the cracking catalyst is deactivated because the process is accompanied by formation of coke which deposits on the catalyst particles. Contaminated catalyst is separated from the cracked hydrocarbon vapors and is further treated with steam to remove hydrocarbon remaining in the pores of the catalyst. The catalyst is then directed into a regenerator where the coke is burned off the surface of the catalyst particles, thus restoring the catalyst's activity and providing the necessary heat for the next reaction cycle. The process of cracking is endothermic. The regenerated catalyst is then used in the new cycle. Typical FCC conditions include a temperature of about 400° C. to about 800° C., a pressure of about 0 to about 688 kPag (about 0 to 100 psig), and contact times of about 0.1 seconds to about 1 hour. The conditions are determined based on the hydrocarbon feedstock being cracked, and the cracked products desired. Zeolite-based catalysts are commonly used in FCC reactors, as are composite catalysts which contain zeolites, silica-aluminas, alumina, and other binders.

Transalkylation is a chemical reaction resulting in transfer of an alkyl group from one organic compound to another. Catalysts, particularly zeolite catalysts, are often used to effect the reaction. If desired, the transalkylation catalyst may be metal stabilized using a noble metal or base metal, and may contain suitable binder or matrix material such as inorganic oxides and other suitable materials. In a transalkylation process, a polyalkylaromatic hydrocarbon feed and an aromatic hydrocarbon feed are provided to a transalkylation reaction zone. The feed is usually heated to reaction temperature and then passed through a reaction zone, which may comprise one or more individual reactors. Passage of the combined feed through the reaction zone produces an effluent stream comprising unconverted feed and product monoalkylated hydrocarbons. This effluent is normally cooled and passed to a stripping column in which substantially all C5 and lighter hydrocarbons present in the effluent are concentrated into an overhead stream and removed from the process. An aromatics-rich stream is recovered as net stripper bottoms, which is referred to as the transalkylation effluent.

The transalkylation reaction can be effected in contact with a catalytic composite in any conventional or otherwise convenient manner and may comprise a batch or continuous type of operation, with a continuous operation being preferred. The transalkylation catalyst is usefully disposed as a fixed bed in a reaction zone of a vertical tubular reactor, with the alkylaromatic feed stock charged through the bed in an upflow or downflow manner. The transalkylation zone normally operates at conditions including a temperature in the range of about 130° C. to about 540° C. The transalkylation zone is typically operated at moderately elevated pressures broadly ranging from about 100 kPa to about 10 MPa absolute. The transalkylation reaction can be effected over a wide range of space velocities. That is, volume of charge per volume of catalyst per hour; weight hourly space velocity (WHSV) generally is in the range of from about 0.1 to about 30 hr⁻¹. The catalyst is typically selected to have relatively high stability at a high activity level.

Alkylation is typically used to combine light olefins, for example mixtures of alkenes such as propylene and butylene, with isobutane to produce a relatively high-octane branched-chain paraffinic hydrocarbon fuel, including isoheptane and isooctane. Similarly, an alkylation reaction can be performed using an aromatic compound such as benzene in place of the isobutane. When using benzene, the product resulting from the alkylation reaction is an alkylbenzene (e.g. toluene, xylenes, ethylbenzene, etc.). For isobutane alkylation, typically, the reactants are mixed in the presence of a strong acid catalyst, such as sulfuric acid or hydrofluoric acid. The alkylation reaction is carried out at mild temperatures, and is typically a two-phase reaction. Because the reaction is exothermic, cooling is needed. Depending on the catalyst used, normal refinery cooling water provides sufficient cooling. Alternatively, a chilled cooling medium can be provided to cool the reaction. The catalyst protonates the alkenes to produce reactive carbocations which alkylate the isobutane reactant, thus forming branched chain paraffins from isobutane. Aromatic alkylation is generally now conducted with solid acid catalysts including zeolites or amorphous silica-aluminas.

The alkylation reaction zone is maintained at a pressure sufficient to maintain the reactants in liquid phase. For a hydrofluoric acid catalyst, a general range of operating pressures is from about 200 to about 7,100 kPa absolute. The temperature range covered by this set of conditions is from about −20° C. to about 200° C. For at least alkylation of aromatic compounds, the temperature range is from about 100° C. to about 200° C. at the pressure range of about 200 to about 7,100 kPa.

Oxidation involves the oxidation of hydrocarbons to oxygen-containing compounds, such as aldehydes. The hydrocarbons include alkanes, alkenes, typically with carbon numbers from 2 to 15, and alkyl aromatics, linear, branched, and cyclic alkanes and alkenes can be used. Oxygenates that are not fully oxidized to ketones or carboxylic acids can also be subjected to oxidation processes, as well as sulfur compounds that contain —S—H moieties, thiophene rings, and sulfone groups. The process is carried out by placing an oxidation catalyst in a reaction zone and contacting the feed stream which contains the desired hydrocarbons with the catalyst in the presence of oxygen. The type of reactor which can be used is any type well known in the art such as fixed-bed, moving-bed, multi-tube, CSTR, fluidized bed, etc. The feed stream can be flowed over the catalyst bed either up-flow or down-flow in the liquid, vapor, or mixed phase. In the case of a fluidized-bed, the feed stream can be flowed co-current or counter-current. In a CSTR the feed stream can be continuously added or added batch-wise. The feed stream contains the desired oxidizable species along with oxygen. Oxygen can be introduced either as pure oxygen or as air, or as liquid phase oxidants including hydrogen peroxide, organic peroxides, or peroxy-acids. The molar ratio of oxygen (O₂) to alkane can range from about 5:1 to about 1:10. In addition to oxygen and alkane or alkene, the feed stream can also contain a diluent gas selected form nitrogen, neon, argon, helium, carbon dioxide, steam or mixtures thereof. As stated, the oxygen can be added as air which could also provide a diluent. The molar ratio of diluent gas to oxygen ranges from greater than zero to about 10:1. The catalyst and feed stream are reacted at oxidation conditions which include a temperature of about 300° C. to about 600° C., a pressure of about 101 kPa to about 5,066 kPa and a space velocity of about 100 to about 100,000 hr⁻¹.

In some processes, all or a portion of the coal feed 10 is mixed with oxygen 85 and steam 90 and reacted under heat and pressure in the gasification zone 20 to form syngas 95, which is a mixture of carbon monoxide and hydrogen. The syngas 95 can be further processed using the Fischer-Tropsch reaction to produce gasoline or using the water-gas shift reaction to produce more hydrogen.

While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims. 

What is claimed is:
 1. A process for pyrolyzing coal using a recycled hydrogen donor, comprising: introducing a coal feed to a pyrolysis zone; heating the coal feed to a temperature of about 300° C. in the absence of hydrogen; introducing a hydrogen donor solvent to the pyrolysis zone after the coal feed is heated to the temperature of about 300° C.; increasing the temperature of the coal feed and the hydrogen donor solvent, while increasing a pressure in the pyrolysis zone to at or above a vapor pressure of the hydrogen donor solvent to evolve coal tar from the pyrolysis zone; increasing the temperature of the coal feed and the hydrogen donor solvent while maintaining the pressure in the pyrolysis zone at or above the vapor pressure of the hydrogen donor solvent to produce a coke stream and a coal tar stream until no further coal tar is evolved from the pyrolysis zone; reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1,000° C. to about 2,000° C.; separating at least an aromatic hydrocarbon rich fraction from the coal tar stream; hydrogenating the aromatic hydrocarbon rich fraction; and, recycling the hydrogenated aromatic hydrocarbon rich fraction to the pyrolysis zone as the hydrogen donor solvent.
 2. The process of claim 1, wherein the hydrogenated aromatic hydrocarbon fraction comprises one or more hydrogenated polycyclic aromatic compounds.
 3. The process of claim 2, wherein the hydrogenated aromatic hydrocarbon fraction comprises one or more of tetralin and decalin.
 4. The process of claim 1, wherein said separating comprises distillation.
 5. The process of claim 1, wherein said separating comprises aromatic extraction.
 6. The process of claim 1, wherein an output volume of the coke stream is controlled based on an amount of the hydrogen donor solvent introduced into the pyrolysis zone relative to an amount of coal feed introduced to the pyrolysis zone.
 7. The process of claim 6, wherein a ratio of the hydrogen donor solvent to the coal feed is set so as to raise a molar ratio of hydrogen to carbon present in the pyrolysis zone into the range of 0.5 to 0.7.
 8. The process of claim 1, wherein reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent comprises reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1,100° C.
 9. The process of claim 1 wherein the aromatic hydrocarbon rich fraction comprises hydrocarbons having an initial boiling point in a range of about 180° C. to about 270° C.
 10. A process for controlling a volume of a coke output during coal pyrolysis, comprising: introducing coal to a pyrolysis zone; heating the coal to a temperature of about 300° C. in the absence of hydrogen; introducing a hydrogen donor solvent to the pyrolysis zone; increasing the temperature of the coal and the hydrogen donor solvent to a temperature of about 475° C., while increasing a pressure in the pyrolysis zone to at or above a vapor pressure of the hydrogen donor solvent; increasing the temperature of the coal and the hydrogen donor solvent to about 600° C. while maintaining the pressure in the pyrolysis zone to produce a coke stream and a coal tar stream; and, reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1,000° C. to about 2,000° C., wherein an amount of the hydrogen donor solvent introduced into the pyrolysis zone relative to an amount of coal introduced to the pyrolysis zone is varied to control an amount of coke produced in the coke stream.
 11. The process of claim 10, wherein a ratio of the hydrogen donor solvent to the coal is set so as to raise the molar ratio of hydrogen to carbon present in the pyrolysis zone into the range of 0.5 to 0.7.
 12. The process of claim 10, wherein the hydrogen donor solvent comprises one or more hydrogenated polycyclic aromatic compounds.
 13. The process of claim 12, wherein the hydrogen donor solvent comprises one or more of tetralin and decalin.
 14. The process of claim 10, further comprising: separating at least an aromatic hydrocarbon rich fraction from the coal tar stream; and, hydrogenating the aromatic hydrocarbon rich fraction.
 15. The process of claim 14 further comprising recycling the hydrogenated aromatic hydrocarbon rich fraction to the pyrolysis zone as the hydrogen donor solvent.
 16. The process of claim 14, wherein said separating comprises distillation.
 17. The process of claim 14, wherein said separating comprises aromatic extraction.
 18. The process of claim 10, wherein reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent comprises reducing the pressure in the pyrolysis zone to approximately atmospheric pressure while increasing the temperature of the coal feed and the hydrogen donor solvent to about 1100° C.
 19. The process of claim 14 wherein the aromatic hydrocarbon rich fraction comprises hydrocarbons having an initial boiling point in a range of about 180° C. to about 270° C.
 20. The process of claim 15 wherein the aromatic hydrocarbon rich fraction comprises hydrocarbons having an initial boiling point in a range of about 180° C. to about 270° C. 